Directionally solidified casting with improved transverse stress rupture strength

ABSTRACT

Directionally solidified columnar grain nickel base alloy casting consisting essentially of, in weight %, of about 11.6% to 12.70% Cr, about 8.50 to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40 to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.015% B, balance essentially Ni and having substantial transverse stress rupture strength and ductility as compared to a similar casting without boron present.

FIELD OF THE INVENTION

[0001] The present invention relates to nickel base superalloy castingsand, more particularly, to directionally solidified (DS) nickel basesuperalloy castings having a columnar grain microstructure andsubstantially improved transverse stress rupture strength and ductility.

BACKGROUND OF THE INVENTION

[0002] U.S. Pat. No. 4,597,809 describes single crystal castings madefrom a nickel base superalloy having a matrix with a compositionconsisting essentially of, in weight %, of 9.5% to 14% Cr, 7% to 11% Co,1% to 2.5% Mo, 3% to 6% W, 1% to 4% Ta, 3% to 4% Al, 3% to 5% Ti, 6.5%to 8% Al+Ti, 0 to 1% Nb, and balance essentially nickel with the matrixcontaining about 0.4 to about 1.5 volume % of a phase based on tantalumcarbide as a result of the inclusion in the alloy of about 0.05% toabout 0.15% C and extra Ta in an amount equal to 1 to 17 times the Ccontent.

[0003] Single crystal castings produced from the aforementioned nickelbase superalloy exhibit inadequate transverse grain boundary strength.The present inventors attempted to produce directionally solidified (DS)columnar grain castings of the nickel base superalloy. However, thedirectionally solidified (DS) columnar gain castings produced wereunacceptable as DS castings as a result of the castings exhibitingessentially no transverse grain boundary strength and no ductility whentested at a temperature of 750 degrees C. (1283 degrees F.) and stressof 660 Mpa (95.7 Ksi). The transverse grain boundary strength andductility were so deficient as to render DS columnar grain castingsproduced from the aforementioned nickel base superalloy unsuitable foruse as turbine blades of gas turbine engines.

[0004] An object of the present invention is to provide DS columnargrain castings based on the aforementioned single crystal nickel basesuperalloy having substantially improved transverse stress rupturestrength and ductility to an extent that the DS castings are acceptablefor use as turbine blades of a gas turbine engine.

[0005] Another object of the present invention is to provide such DScolumnar grain castings based on the aforementioned single crystalnickel base superalloy having substantially improved transverse stressrupture strength and ductility without adversely affecting othermechanical properties and corrosion resistance of the DS castings.

SUMMARY OF THE INVENTION

[0006] The present invention involves including boron in the nickel basesuperalloy described hereabove in a manner discovered to significantlyimprove transverse stress rupture strength and ductility ofdirectionally solidified (DS) columnar grain castings produced from theboron modified superalloy. In accordance with the present invention,boron is added to the aforementioned superalloy composition in aneffective amount to substantially improve transverse stress rupturestrength and ductility of directionally solidified columnar graincastings produced from the boron-modified superalloy. The boronconcentration preferably is controlled in the range of about 0.003% toabout 0.015% by weight of the superalloy composition to this end. Inconjunction with addition of boron to the superalloy composition, thecarbon concentration preferably is controlled in the range of about0.05% to about 0.11% by weight of the superalloy composition.

[0007] A preferred nickel base superalloy in accordance with anembodiment of the present invention consists essentially of, in weight%, of about 11.6% to 12.70% Cr, about 8.50 to 9.5% Co, about 1.65% to2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40 to3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003%to 0.015% B, and balance essentially Ni. The boron modified nickel basesuperalloy can be cast as DS columnar grain castings pursuant toconventional DS casting techniques such as the well known Bridgeman moldwithdrawal technique.

[0008] DS castings produced in this manner typically have a plurality ofcolumnar grains extending in the direction of the principal stress axisof the casting with the <001> crystal axis generally parallel to theprincipal stress axis. DS columnar grain castings pursuant to thepresent invention preferably exhibit a stress rupture life of at leastabout 150 hours and elongation of at least about 2.5% when tested at atemperature of 750 degrees C. (1283 degrees F.) and stress of 660 Mpa(95.7 Ksi) and will find use as turbine blades, vanes, outer air sealsand other components of a industrial and aero gas turbine engines.

[0009] The above objects and advantages of the present invention willbecome more readily apparent form the following detailed descriptiontaken with the following drawings.

DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1A is a photomicrograph at 11.25× taken transverse to thelongitudinal axis of a DS cast specimen showing the columnar grainmicrostructure.

[0011]FIGS. 1B, 1C and 1D are similar photomicrographs at 50×, 100× and200×, respectively, of the columnar grain microstructure of FIG. 1A.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention involves including boron in a particularnickel base superalloy in a manner discovered to unexpectedly andsurprisingly provide significantly enhanced transverse stress rupturestrength and ductility of DS columnar grain castings produced from theboron-modified superalloy. The nickel base superalloy which is modifiedpursuant to the present invention is described in U.S. Pat. No.4,597,809, the teachings of which are incorporated herein by reference.A nickel base superalloy in accordance with an embodiment of theinvention consists essentially of, in weight %, of about 9.5% to 14% Cr,about 7% to 11% Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1% to6% Ta, about 3% to 4% Al, about 3% to 5% Ti, about 0 to 1% Nb, andbalance essentially Ni and B present in an amount effective tosubstantially improve transverse stress rupture strength of a DS castingas compared to a similar casting without boron present.

[0013] The present invention modifies the aforementioned nickel basesuperalloy to include boron in the alloy in an amount discoveredeffective to provide substantial transverse stress rupture strength andductility of a DS columnar grain casting produced from the alloy ascompared to a similar casting without boron present. Preferably, thenickel base superalloy is modified by the inclusion of boron in therange of about 0.003% to about 0.015%, preferably 0.010% to 0.015%, byweight of the superalloy composition to this end. In conjunction withaddition of boron to the superalloy composition, the carbonconcentration is controlled in a preferred range of about 0.05% to about0.11% by weight of the superalloy composition. The transverse stressrupture strength and ductility of DS castings produced from the boronmodified nickel base superalloy are provided to an extent that thecastings are rendered acceptable for use as turbine blades and othercomponents of gas turbine engines.

[0014] A particularly preferred boron-modified nickel base superalloycasting composition in accordance with the present invention consistsessentially of, in weight %, of about 11.6% to 12.70% Cr, about 8.50 to9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to5.20% Ta, about 3.40 to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to0.11% C, about 0.003% to 0.015% B, and balance essentially Ni andcastable to provide a DS columnar grain microstructure. The DSmicrostructure of the columnar grain casting, FIG. 1A, typicallyincludes about 0.4 to about 1.5 volume % of a phase based on tantalumcarbide shown as light gray particles in FIGS. 1B, 1C and 1D. Some ofthe light gray particles in the DS microstructure appear to be eutecticgamma prime phase. The somewhat rounded dark features dispersedthroughout the DS microstructure in FIGS. 1A through 1D comprise voidspresent in the particular cast specimens examined. Although not wishingto be bound by any theory, it is thought that boron and carbon tend tomigrate to the grain boundaries in the DS microstructure to add strengthand ductility to the grain boundaries at high service temperatures, forexample, 816 degrees C. (1500 degrees F.) typical of gas turbine engineblades.

[0015] DS columnar grain castings produced from the above boron-modifiednickel base superalloy in accordance with the present inventiontypically have the <001> crystal axis parallel to the principal stressaxis of the casting and exhibit a stress rupture life of at least about150 hours and elongation of at least about 2.5% when tested at atemperature of 750 degrees C. (1283 degrees F.) and stress of 660 Mpa(95.7 Ksi) applied perpendicular to the <001> crystal axis of thecasting.

[0016] For example, the following DS casting tests were conducted andare offered to further illustrate, but not limit, the present invention.A heat #1 having a nickel base superalloy composition in accordance withthe aforementioned U.S. Pat. No. 4,597,809 and heats #1A and #2 of boronmodified nickel base superalloy in accordance with the present inventionwere prepared with the following compositions, in weight percentages,set forth in Table I: TABLE I Heat Cr Co Mo W Ta Al Ti C B Ni #1 12.19.0 1.8 3.7 5.2 3.6 4.0 0.7 0.001 balance #1A 12.1 9.0 1.8 3.7 5.2 3.64.0 0.8 0.010 balance #2 12.1 9.0 1.8 3.7 5.2 3.6 4.0 0.9 0.011 balance

[0017] Each heat was cast to form DS columnar grain non-cored castingshaving a rectangular shape for transverse stress rupture testingpursuant to ASTM E-139 testing procedure. The DS castings were producedusing the conventional Bridgeman mold withdrawal directionalsolidification technique. For example, each heat was melted in acrucible of a conventional casting furnace under a vacuum of 1 micronand superheated to 1427 degrees C. (2600 degrees F.). The superheatedmelt was poured into an investment casting mold having a facecoatcomprising zircon backed by additional slurry/stucco layers comprisingzircon/alumina. The mold was preheated to 1482 degrees C. (2700 degreesF.) and mounted on a chill plate to effect unidirectional heat removalfrom the molten alloy in the mold. The melt-filled mold on the chillplate was withdrawn from the furnace into a soldification chamber of thecasting furnace at a vacuum of 1 micron at a withdrawal rate of 6-16inches per hour. The DS columnar grain castings were cooled to roomtemperature under vacuum in the chamber, removed from the mold inconventional manner using a mechanical knock-out procedure, heat treatedat 1250 degrees C. (2282 degrees F.) for 4 hours, analyzed forchemistry, and machined to specimen configuration. Stress rupturetesting was conducted in air at a temperature of 750 degrees C. (1283degrees F.) and stress of 660 Mpa (95.7 Ksi) applied perpendicular tothe <001> crystal axis of the specimens.

[0018] The results of stress rupture testing are set forth in TABLE IIbelow where LIFE in hours (HRS) indicates the time to fracture of thespecimen, ELONGATION is the specimen elongation to fracture, and RED OFAREA is the reduction of area of the specimens to fracture. The BASELINEdata corresponds to test data for Heat #1, and the #1A and #2 datacorresponds to test data for Heat #1A and #2, respectively. The BASELINEdata represent an average of two stress rupture test specimens, whilethe #1A and #2 data represent a single stress rupture test specimen.TABLE II # OF TEMPERATURE- STRESS- ELONGATION RED OF ALLOY TESTS C. (F.)Mpa (KSI) LIFE (HRS) (%) AREA (%) BASELINE 2 750 (1382) 660 (95.7) 0 0 0#2 1 750 (1382) 660 (95.7) 182 2.6 6.3 #1A 1 750 (1382) 660 (95.7) 2753.1 4.7

[0019] It is apparent from TABLE II that the DS columnar grain specimensproduced from heat #1 exhibited in effect essentially no (e.g. zerohours stress rupture life) transverse grain boundary strength whentested at a temperature of 750 degrees C. (1283 degrees F.) and stressof 660 Mpa (95.7 Ksi). That is, the specimens failed immediately toprovide an essentially zero stress rupture life. Moreover, theelongation and reduction of area data were essentially zero. Thesestress rupture properties are so deficient as to render the DS columnargrain castings produced from heat #1 unacceptable for use as turbineblades of gas turbine engines.

[0020] In contrast, TABLE II reveals that DS columnar grain specimensproduced from heat #1A exhibited a stress rupture life of 275 hours, anelongation of 3.1%, and a reduction of area of 4.7% and specimens fromheat #2 exhibited a stress rupture life of 182 hours, an elongation of2.6%, and a reduction of area of 6.3% when tested at a temperature of750 degrees C. (1283 degrees F.) and stress of 660 Mpa (95.7 Ksi). Thesestress rupture properties of the invention represent an unexpected andsurprising improvement over those of specimens produced from heat #1 andrender DS columnar grain castings produced from heats #1A and #2 moresuitable for use as turbine blades and other components of gas turbineengines.

[0021] The present invention is effective to provide DS columnar graincastings with substantial transverse stress rupture strength andductility. These properties are achieved without adversely affectingother mechanical properties, such as tensile strength, creep strength,fatigue strength, and corrosion resistance of the DS castings. Thepresent invention is especially useful to provide large DS columnargrain industrial gas turbine (IGT) blade castings which have the alloycomposition described above to impart substantial transverse stressrupture strength and ductility to the castings and which have a lengthof about 20 centimeters to about 60 centimeters and above, such as about90 centimeters length, used throughout the stages of the turbine ofstationary industrial gas turbine engines. The above describedboron-modified nickel base superalloy casting composition can be cast asDS columnar grain or single crystal components.

[0022] While the invention has been described in terms of specificembodiments thereof, it is not intended to be limited thereto but ratheronly to the extent set forth in the following claims.

We claim:
 1. A directionally solidified columnar grain nickel base alloycasting, consisting essentially of, in weight %, of about 9.5% to 14%Cr, about 7% to 11% Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1%to 6% Ta, about 3% to 4% Al, about 3% to 5% Ti, about 0 to 1% Nb, andbalance essentially Ni and B present in an amount effective tosubstantially improve transverse stress rupture strength of said castingas compared to a similar casting without boron present.
 2. The castingof claim 1 wherein B is present in the range of about 0.003% to about0.015% by weight.
 3. The casting of claim 1 that has a stress rupturelife of at least about 150 hours and elongation of at least about 2.5%when tested at a temperature of 750 degrees C. (1283 degrees F.) andstress of 660 Mpa (95.7 Ksi) applied in a direction perpendicular to a<001> crystal axis of said casting.
 4. The casting of claim 1 which isgas turbine engine blade having a length of about 20 centimeters toabout 90 centimeters.
 5. A directionally solidified columnar grainnickel base alloy casting consisting essentially of, in weight %, ofabout 11.6% to 12.70% Cr, about 8.50 to 9.5% Co, about 1.65% to 2.15%Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40 to 3.80%Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to0.015% B, balance essentially Ni and having substantially improvedtransverse stress rupture strength as compared to a similar castingwithout boron present.
 6. The casting of claim 5 that has a stressrupture life of at least about 150 hours and elongation of at leastabout 2.5% when tested at a temperature of 750 degrees C. (1283 degreesF.) and stress of 660 Mpa (95.7 Ksi) applied perpendicular to a <001>crystal axis of said casting.
 7. A directionally solidified columnargrain nickel base alloy casting having a nominal composition consistingessentially of, in weight %, of about 12.00% Cr, about 9.00% Co, about1.85% Mo, about 3.70% W, about 5.10% Ta, about 3.60% Al, about 4.00% Ti,about 0.0125% B, about 0.09% C, balance essentially Ni and having astress rupture life of at least about 150 hours and elongation of atleast about 2.5% when tested at a temperature of 750 degrees C. (1283degrees F.) and stress of 660 Mpa (95.7 Ksi) applied perpendicular to a<001> crystal axis of said casting.
 8. A method of making adirectionally solidified casting, comprising casting an alloy consistingessentially of, in weight %, of about 9.5% to 14% Cr, about 7% to 11%Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1% to 6% Ta, about 3%to 4% Al, about 3% to 5% Ti, about 0 to 1% Nb and balance essentially Niand B in an amount effective to substantially improve transverse stressrupture strength into a mold, and directionally solidifying the alloy inthe mold to form a columnar grain casting having substantially improvedtransverse stress rupture strength by virture of the inclusion of boronin said alloy as compared to a similar casting without boron present. 9.The method of claim 8 wherein B is included in an amount of about 0.003%to about 0.015% by weight.
 10. The method of claim 8 wherein thedirectionally solidified casting has a stress rupture life of at leastabout 150 hours and elongation of at least about 2.5% when tested at atemperature of 750 degrees C. (1283 degrees F.) and stress of 660 Mpa(95.7 Ksi) applied perpendicular to a <001> crystal axis of saidcasting.
 11. A method making a directionally solidified casting,comprising providing a nickel base alloy consisting essentially of, inweight %, of about 11.6% to 12.70% Cr, about 8.50 to 9.5% Co, about1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about3.40 to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about0.003% to 0.015% B, and balance essentially Ni, casting the alloy into amold, and solidifying the alloy in the mold to form a casting having adirectionally solidified columnar grain mcirostructure havingsubstantially improved transverse stress rupture strength as compared toa similar casting without boron present.
 12. Nickel base alloyconsisting essentially of, in weight %, of about 11.6% to 12.70% Cr,about 8.50 to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W,about 4.80% to 5.20% Ta, about 3.40 to 3.80% Al, about 3.9% to 4.25% Ti,about 0.05% to 0.11% C, about 0.003% to 0.015% B, balance essentiallyNi.